Milton calls to his muse to "sing" of...
Of Man's first disobedience, and the fruit
Of that forbidden tree whose mortal taste
Brought death into the World, and all our woe,
With loss of Eden, till one greater Man
Restore us, and regain the blissful seat...
He's asking the muses, who traditionally inspire poets, to bring him the inspiration to write about these grand themes, and outlines the plot he is hoping to write: man's banishment from heaven and the apple, death's presence in the world, the loss of Eden, and then the redemption of Christ.
Milton admits that he needs the muses' "aid", as his themes will be extremely complex to pull off:
Invoke thy aid to my adventurous song,
That with no middle flight intends to soar
Above th' Aonian mount, while it pursues
Things unattempted yet in prose or rhyme.
Finally, Milton comments that - at the very end of this first section - he hopes he can tell his story so well that it does justify to the men who reader it why God did what he did:
That, to the height of this great argument,
I may assert Eternal Providence,
And justify the ways of God to men.